1. Field of the Invention
This invention relates to a tag device, an antenna, and a portable card and, more particularly, to a tag device for performing radio communication, an antenna for radiating and receiving radio waves, and a portable card for performing radio communication.
2. Description of the Related Art
In recent years attention has been riveted on an automatic identification technique called radio frequency-identification (RFID). The RFID is a kind of radio communication system by which an object on which an IC tag is put is automatically identified by radio on a noncontact basis. With the RFID, IC tags can be put on all objects and be connected to networks. Accordingly, the development of the RFID has advanced swiftly as a technique which is effective in building the next generation of ubiquitous network society.
The RFID system includes a reader/writer and an IC tag. By performing radio communication via each antenna, information is written from the reader/writer to the IC tag or information stored in the IC tag is read out.
Conventionally, the 13.56 MHz frequency band or the 2.45 GHz frequency band has been used in the RFID. However, in recent years attention has been riveted on IC tags using an ultrahigh-frequency (UHF) band (300 to 3,000 MHz).
Such IC tags use 952 to 954 MHz in particular in the UHF band for performing communication. Compared with existing IC tags using the 13.56 MHz frequency band or the 2.45 GHz frequency band, communication distance to readers/writers can be lengthened and areas in which communication can be performed can be widened.
On the other hand, an IC tag does not include a power source (battery). Accordingly, when a reader/writer communicates with an IC tag, the IC tag obtains electric power from radio waves radiated from the reader/writer or a magnetic field produced by the reader/writer.
That is to say, roughly speaking, there are two methods for generating electric power in the RFID. One is a radio wave method and the other is an electromagnetic induction method. With an IC tag using the 2.45 GHz frequency band or a UHF band, the radio wave method in which radio waves radiated from a reader/writer are converted into electric power is adopted. With an IC tag using the 13.56 MHz frequency band, the electromagnetic induction method in which electric power is obtained from a magnetic field produced near an antenna of a reader/writer is adopted.
FIG. 12 is a view showing the concept of the radio wave method. An RFID system 100 which performs communication by using the 2.45 GHz frequency band or a UHF band comprises a reader/writer 110 and an IC tag 120. The IC tag 120 includes an antenna 121, a rectifying circuit 122, and a control circuit 123.
When the IC tag 120 receives radio waves sent from the reader/writer 110 via the antenna 121, the rectifying circuit 122 rectifies the radio waves which are alternating current signals into direct current signals. The direct current signals are used as a power source and are applied to the control circuit 123 which exercises modulation/demodulation control and logical control.
FIG. 13 is a view showing the concept of the electromagnetic induction method. An RFID system 200 which performs communication by using the 13.56 MHz frequency band includes a loop antenna 210 of a reader/writer and a loop antenna 220 of an IC tag (loop antenna has a structure in which a conductor is looped in the shape of a circle or a square).
It is assumed that the loop antennas 210 and 220 are in close proximity to each other. If an electric current ia is passed counterclockwise through the loop antenna 210 of the reader/writer, an upward magnetic field H1 is produced as shown in FIG. 13. Just then, an electric current ib runs clockwise through the loop antenna 220 of the IC tag so as to cancel out the magnetic field H1, and a downward magnetic field H2 is produced.
With an IC tag using the 13.56 MHz frequency band, a loop antenna is used and an electric current is generated in this way by a magnetic field produced because of electromagnetic induction. As a result, electric power can be obtained. (In the 13.56 MHz frequency band, the loop antenna is used for making electromagnetic induction occur. However, the loop antenna itself is used not only for making electromagnetic induction occur but also for radiating ordinary radio waves.)
With the radio wave method described in FIG. 12, as the frequency of the radio waves becomes lower (wavelength of the radio waves becomes longer), longer-distance communication can be performed. Accordingly, on simple consideration it turns out that the communication distance of an IC tag using a UHF band is at least about three times the communication distance of an IC tag using the 2.45 GHz frequency band.
With the electromagnetic induction method described in FIG. 13, on the other hand, the loop antenna of the reader/writer and the loop antenna of the IC tag must be in close proximity to each other. When the IC tag using the 13.56 MHz frequency band is distant from the reader/writer, the magnetic field weakens sharply and electric power cannot be secured.
Specific communication distance of an IC tag using the 13.56 MHz frequency band is 70 to 80 cm maximum and specific communication distance of an IC tag using the 2.45 GHz frequency band is about 2 m maximum. Experiments showed that specific communication distance of an IC tag using a UHF band is about 7 m (theoretical value is about 10 m).
Basically dipole antennas (folded dipole antennas, for example) with a length of λ/2 (λ is the wavelength of radio waves) are used as antennas of IC tags using the 2.45 GHz frequency band or a UHF band.
The length of an antenna of an IC tag using each frequency band will now be calculated. λ (wavelength)=C (velocity of light)/f (frequency), so the length of an antenna of an IC tag using the 2.45 GHz frequency band is calculated in the following way. 3×108/(2.45×109)≈0.122 (m) Half of 0.122 (m) is about 6 (cm). Therefore, the length of the antenna of the IC tag using the 2.45 GHz frequency band is about 6 cm.
Similarly, the length of an antenna of an IC tag using a UHF (953 MHz, for example) frequency band is calculated. 3×108/(953×106)≈0.3 (m) Half of 0.3 (m) is about 15 (cm). That is to say, the length of the antenna of the IC tag using the 953 MHz frequency band is about 15 cm. (Therefore, to simply obtain calculated communication distance on the basis of antenna length without changing electrical length, the IC tag using the 2.45 GHz frequency band should hold an antenna with a length of 6 cm minimum and the IC tag using the 953 MHz frequency band should hold an antenna with a length of 15 cm minimum.)
It is assumed that an IC tag using the 13.56 MHz frequency band uses a dipole antenna for obtaining electric power by the radio wave method. 3×108/(13.56×106)≈22 (m) Half of 22 (m) is about 11 (m). Therefore, the length of the dipole antenna is about 11 m. This length is not practical. For this reason, an IC tag using the 13.56 MHz frequency band obtains electric power not by the radio wave method but by the electromagnetic induction method.
The disadvantage of IC tags using the 13.56 MHz frequency band is that communication distance cannot be lengthened. On the other hand, the communication distance of IC tags using the 2.45 GHz frequency band is 2 m, so they are practical in a fairly wide range. With IC tags using the 2.45 GHz frequency band, however, radio waves at a frequency of 2.45 GHz are intercepted or absorbed if a liquid, such as water or alcohol, is near the IC tags (2.45 GHz is the same as the frequency of electronic ovens).
With IC tags using a UHF band, on the other hand, communication distance is long and the disadvantage of IC tags using the 2.45 GMHz frequency band does not exist. Accordingly, compared with the case where IC tags using existing frequency bands are used, it is easy for a reader/writer to read a plurality of IC tags in the block. In addition, radio waves in a UHF band are diffracted significantly, so an IC tag which cannot be seen from the reader/writer can be read.
IC tags using a UHF band have many such advantages and much hope is placed on them. However, under the present conditions the realization of efficient RFID services in an environment in which IC tags using a UHF band and the existing 13.56 MHz and 2.45 GMHz frequency bands are included is desired.
Conventionally, an IC tag coated with a thin flexible protection laminate is disclosed (see, for example, Japanese Unexamined Patent Publication No. 8-88586 (paragraphs [0018]-[0021] and FIG. 3)).
If the RFID is used, services may widely be provided by the use of IC tag cards. For example, portable cards in which IC tags are embedded are used for managing the alighting and boarding of users of railroads or airplanes or making purchases in department stores and the like.
Conventionally, folded dipole antennas have been used in many IC-tags using a UHF band.
FIG. 14 is a view showing a folded dipole antenna. A dipole antenna dp radiates radio waves when high-frequency waves are supplied from a power feeding section (wave source) located in the middle of a conductor. The dipole antenna dp is a linear antenna of the most basic type (antenna length is basically λ/2). A folded dipole antenna fdp has a structure in which a conductor corresponding to one wavelength is folded back with the dipole antenna dp as a basis.
Usually a card is carried in a person's breast pocket or is put over a reader/writer with a person's hand. That is to say, there are many cases where a card is touching a person's body. However, when a card in which an IC tag using a UHF band and including the above folded dipole antenna fdp is embedded is near a person's body, radio waves radiated are intercepted or absorbed by the person's body. As a result, the radio wave radiation and receiving characteristics deteriorate.
FIG. 15 is a view showing a problem with a conventional card using a UHF band. In FIG. 15, the dipole antenna dp is near a person's body. (The basic principles of the folded dipole antenna fdp are the same as those of the dipole antenna dp, so for the sake of simplicity the dipole antenna dp is shown and described.)
Usually an electric current i1 shown in FIG. 15 runs through the dipole antenna dp and radio waves v are radiated. If human skin or the like is near the dipole antenna dp, then an electric current i2 is generated on the surface of the person's body which is a conductor. The electric currents i1 and i2 run in opposite directions. As a result, the electric currents i1 and i2 cancel out each other. Accordingly, it is difficult for the electric current i1 to run through the dipole antenna dp. This means that the radio waves v are not fully radiated.
Ground plane antennas which are plane antennas are also used widely in IC tag cards regardless of used frequency bands such as a UHF band.
FIG. 16 is a view showing a ground plane antenna. A ground plane antenna 300 has a structure in which a GND plate (ground plane) 302 is located on one surface (reverse) of a dielectric substrate 301 and in which a radiation element 303 is located on the other surface (front) of the dielectric substrate 301. (High-frequency signals are supplied by, for example, connecting an internal conductor (core) of a coaxial cable (not shown) to the radiation element 303 and connecting an external conductor of the coaxial cable to the GND plate 302.)
The ground plane antenna 300 has the two side of the reverse and the front. If the front of the ground plane antenna 300 on which the radiation element 303 is located is turned toward a person's body, the same phenomenon that was described in FIG. 15 occurs. Accordingly, with the ground plane antenna 300, the radio waves v radiated are intercepted or absorbed by the person's body. As a result, the radio wave radiation and receiving characteristics deteriorate.
As stated above, under the present conditions RFID services are provided by performing communication not by the use of a single frequency band but by the use of a plurality of frequency bands including a UHF band and the 13.56 MHz frequency band.
Therefore, some cards use UHF bands and others use the 13.56 MHz frequency band. When a person carries cards, the cards using different frequency bands will be put in a wallet, a commutation-ticket holder, or the like.
As described in FIG. 13, however, the electromagnetic induction method is adopted for an IC tag included in a card using the 13.56 MHz frequency band. If a second card using a UHF band or 2.45 GHz frequency band is placed on the front or the back of a loop antenna of the IC tag in a wallet or the like, then a magnetic field produced by the loop antenna cannot pass through due to obstruction by an IC tag included in the second card. This means that an electric current does not run through the loop antenna. As a result, the IC tag using the 13.56 MHz frequency band does not function and communication cannot be performed.
For example, a card using the 13.56 MHz frequency band and a card using a UHF band are put-in a commutation-ticket holder, one over the other. If the card using the 13.56 MHz frequency band is put over a reader/writer in this state, then radio waves to be radiated or received by the card using the 13.56 MHz frequency band are obstructed by a conductor portion in the card using the UHF band.